Rather than hardcoding the path to GHC's libdir which may change, use
`ghc --print-libdir` in wrappers.
Another option would be to expose mkGhcLibdir from generic-builder.nix.
Also add a test for mueval.
Update all usage of lib.concatStrings (lib.intersperse ...) to
lib.concatStringsSep. This produces the same result as per https://github.com/NixOS/nixpkgs/pull/135843,
however it yields a performance benefit on Nix versions that
support the builtins.concatStringsSep primop.
